Spatio‐temporal variability of suspended sediments in rivers and ecological implications of reservoir flushing operations

Abstract

High suspended sediment concentrations during reservoir flushing are known to be harmful to biota in downstream river stretches. Therefore, it is common practice to set legal concentration limits for upstream reservoir management operations such as flushings or controlled drawdowns. However, as shown by measurements, there is a considerable spatio-temporal variability of suspended sediment concentrations both in the longitudinal profile of rivers and in river cross-sections. To consider this variability in management operations, SED-FISH-a three-dimensional modelling approach-was developed to study this variability in a wider context by upscaling cross-sectional measurements of suspended sediments to high-resolution threedimensional information on the reach scale in an alpine river. The resulting patterns of suspended sediment concentrations were integrated over their respective time of occurrence for various scenarios in order to calculate severity of harmful impacts for target fish species. The modelling results identified refugial habitats with reduced negative impacts in near-bank zones even for relatively high suspended sediment concentrations in the centre of the river. Moreover, a substantially larger variability of both suspended sediment concentrations and associated harmful impacts was found for a winding riverbed morphology as compared with a straight reach. Both these findings and the developed modelling tool could assist in establishing individual case-based concentration limits for reservoir management operations in the future and should also be taken into account when planning river regulation or restoration measures.

Controlled Reservoir Drawdown—Challenges for Sediment Management and Integrative Monitoring: An Austrian Case Study—Part B: Local Scale

“…However, to guarantee operation even during low flow these instruments need to be placed close to the river bed. Thus, cross sectional variability in SSC is often neglected; however especially fish can response in terms of active movement to areas with lower concentrations or even into refugial habitats, such as tributaries [25,26]. Such lack of standards are related to the fact that only a few basic studies, which investigated SSC variability on a habitat basis during reservoir flushes or controlled drawdowns, are available.…”

HEM Impoundment—A Numerical Prediction Tool for the Water Framework Directive Assessment of Impounded River Reaches

“…Adaptive sediment management would include dredging as well as flushing and sluicing. However, based on numerical sediment transport modelling [31,55,56], threshold conditions for both active management and self-forming equilibrium status may be predicted and evaluated. Hence, improvement of various, small hydropower plant sites will be possible using the novel predictive HEM Impoundment Tool even if there is only a re-classification of the risk-assessment for failing the good ecological status, according to the EU Water Framework Directive.…”